This invention relates to an analog-to-digital (hereinafter abbreviated as "A/D") conversion circuit and, more particularly, to an A/D conversion circuit capable of preventing deterioration in the signal-to-noise ratio due to error occurring in A/D conversion means during application of a signal of a relatively small level and thereby preventing reduction in the dynamic range.
In conventional A/D conversion circuits, an analog input is directly converted to a digital signal irrespective of the level of the input signal.
Currently available A/D converters for household uses are not necessarily of an excellent linearity but their accuracy in conversion actually is even poorer than is generally expected. A 16-bit A/D converter, for example, has actually an accuracy of 14 bits and 2 bits of lower orders contain much conversion error. For this reason, the smaller the level of the analog input, the worse is the condition in which the A/D converter is used with resulting deterioration in the signal-to-noise ratio and, consequently, reduction in the dynamic range.
It is, therefore, an object of the invention to provide an A/D conversion circuit which has eliminated the above described disadvantage of the prior art A/D conversion circuit and is capable of preventing deterioration in the signal-to-noise ratio during application of a signal of a small level and thereby preventing reduction in the dynamic range.